The opposite of the low side switch is the high side switch. While you can use a JFET for this circuit, an enhancement mode MOSFET works better. Saturation means there is enough base current to turn on the transistor fully.įor an N-Channel MOSFET, the source connects to ground, and the drain connects to the negative side of the load. As a switch, the BJT operates in saturation mode. Low-Side Transistor Examples (Note the FET has a pull-down resistor.)įor an NPN BJT, the emitter connects to ground, and the collector attaches to the negative side of the load. Typically these use an NPN BJT or an N-Channel MOSFET. Since the transistor is switching the path to ground or is sitting on the low side of the load, it is called a low side switch. When the transistor is connected to ground, that means the load is between +V and the transistor.

I have a couple of videos I made and some from element14’s The Learning Circuit which do a great job introducing transistors. If you are new to transistors, check out the resource links at the bottom.

In this post, I draw the configuration for both transistor types, talk about which requires a driver, and explain why you would use either. Either transistor configuration can use a BJT or MOSFET. The location of the transistor determines the type of circuit and its name. There are two configurations for a transistor switch: low side and high side. A common task for a transistor is switching a device on and off.

Transistor as a switch